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Community Succession. Universal process of directional change in vegetation during ecological time. Recognized by a progressive change in the species composition of the community

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Community succession l.jpg
Community Succession

  • Universal process of directional change in vegetation during ecological time.

    • Recognized by a progressive change in the species composition of the community

  • When an area is stripped of its vegetation by fire, flood, glaciation, or volcanic activity, it doesn’t take long for new vegetation to grow.

  • Three stages – Primary succession, secondary succession, climax community.


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Species Area Curves opportunistic species.

  • Species area curve predicts that larger islands will have more species than smaller islands.

  • S=cAzwhere

    • S = number of species

    • c = a constant measuring the number of species per unit area

    • A = area of island (in square units)

    • z = a constant measuring the slope of the line relating to S and A


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Island Biogeography opportunistic species.

  • Number of species (on a plot or island) is a balance between immigration and extinction.

  • If immigration exceeds extinction, then the number of species will increase.

  • Number of species usually at equilibrium


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Island Biogeography opportunistic species.

  • Immigration rates on islands are related to the distance from the mainland.

    • Close islands have greater immigration rates than far islands

  • Extinction rates on islands are related to the size of the island.

    • Extinction rates are greater for small islands than for large islands


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Island Biogeography opportunistic species.


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Neotropical Migratory Birds opportunistic species.

  • Neotropical = ‘New Tropics’

    • New World vs old world

    • Western Hemisphere

  • Neotropical birds breed in Canada and the United States during the northern hemisphere’s summer and spends the rest of the year in the tropics.

    • Defined as a species in which the majority of individuals breed north of the Tropic of Cancer (latitude = 23 degrees north)

    • About 200 species


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Migration Distance opportunistic species.

  • Varies across species and within species

  • Shortest (a few hundred miles) are those birds that breed in the southern US and overwinter in Mexico.

  • Some of the longest are birds that breed in the arctic tundra in northernmost Canada and winter as far south as the southermost tip of South America

    • One way mileage = 10,000

  • Arctic Tern

    • Nests as far north as land extends

    • Overwinters near the south pole

    • Sees more daylight than any other species

    • Round trip covers about 22,000 miles.


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Why Migrate? opportunistic species.

  • They can take advantage of seasonally abundant food supply and avoid times and places that food supply is low.

    • Flying insects, caterpillars, fruits and nectar are abundant during our spring and summer, but not winter.

  • Ultimate reason is breeding success.

    • Can raise more young if they migrate than if they stayed in the tropics.

    • Abundant protein-rich food, longer daylight hours, more room, possibly fewer predators.


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When To Migrate? opportunistic species.

  • Internal clock controls the onset of migration and the premigration preparations.

  • Environmental factors control this clock

    • Certain changes in a bird’s environment stimulate the production of certain hormones, which leads to changes in behavior and physiology.

    • Change in day length for example


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How To Get There? opportunistic species.

  • Short migraters and waterfowl generally learn breeding and wintering locations from older more experienced birds

    • Often family members

  • Most long distance migraters are genetically programmed to make the trip.

    • First migration is completely under genetic control

    • Subsequent trips may incorporate previous experiences (return each year to good reproductive grounds)


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Migration Routes opportunistic species.

  • Follow land through Mexico into the United States

  • Cross the Gulf of Mexico

    • First/last encountered land important to survival

    • Rest and refueling


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Seasonal Habitats opportunistic species.

  • Wintering Grounds

    • Sufficient food for premigration preparations

  • Migratory Habitat

    • Fat reserves, nutrients, vulnerability to predation

  • Breeding Grounds

    • Reproductive success


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Habitat Variety Important opportunistic species.

  • With the diversity of migratory birds, a diversity of habitats is needed in the migratory habitats.

    • Reduces competition


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Page 496 opportunistic species.

Top Down or Bottom Up?

Bottom Up Control resources control community

N  V  H  P

Top Down Control  Predators control the community

N  V  H  P

Top down control = Trophic Cascade Model

Freshwater Pond For Example:

Phytoplankton  Zooplankton  Small Fish  Large Fish

Remove large fish then small fish increase, zooplankton decreases and phytoplankton increases.

Effects will be propagated up and down food chain as a +/-


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Page 471 opportunistic species.

Keystone Species

  • A species that occupies a specific niche that is extremely important in determining community structure.

    • When that species is removed, the community dramatically changes

    • Not typically the most common species in a community


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Pisaster ochraceous opportunistic species. (a starfish)

  • Keystone species in the rocky intertidal communities of western North America.

  • Is a strong predator for a mussel (Mytilus californianus)

    • The starfish can not eat large mussels, so the mussels have a size-related refuge from predation

    • This mussel can out-compete other invertebrates for space, but the starfish takes away that competitive edge.

  • When the starfish were removed, mussel numbers increased and excluded other invertebrates and algae from attachment sites.


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Sea Otters opportunistic species.

  • Key Stone Predator in North Pacific

    • Once extremely abundant, reduced to near extinction in the early 1900’s by the fur trade

    • Feed heavily on sea urchins and thus can control their populations

  • Sea urchins feed heavily on macroalgae (kelp) and where sea urchin abundance is high, kelp is basically nonexistent

  • Where sea urchin abundance is low, kelp is common along with all of the other species associated with it.


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Case Study opportunistic species.

  • Sea otters have declined (sometimes 25% per year) in Alaska since about 1990, and the kelp beds have begun to disappear as sea urchins increased.

  • Killer whales are suspected because their prey base (seals, sea-lions) has declined, and their predation on sea otters has increased.

  • Seals and Sea-lion population declines have been attributed to a decline in their food base (fish).

  • Fish declines have been attributed to overharvesting in the North Pacific.

  • So, overharvesting of fish may have led to a cascade of events that were unexpected.